The present invention relates to a BICMOS Content Addressable Memory sense amplifier that combines the high drive capability and switching time of bipolar devices with the low noise and power dissipation of CMOS devices.
Content Addressable Memories (CAMs) consist of rows of CAM cells. Typically, each CAM cell consists of a static Random Access Memory (RAM) cell to store a data bit, paired with a comparator to determine bit-wise matching between stored data and input data. The compare logic within each CAM cell determines a response of the cell output. For example, the convention employed for the CAM cells of the following description is: when an input bit does not match with the stored bit in a cell, the output of that cell is pulled high, and when there is a match the output remains low. The outputs of all cells in each row connect to a single match line, and each match line is fed into a sense amplifier whose function is to detect a word match. Therefore, during each cycle an input word is clocked in and gets compared to every row of the CAM. In each case where there is a non-matching bit, the match line for that entire row is pulled high. A match line that has not changed indicates a match between all bits of the input word and the contents of that particular CAM row.
FIG. 1 illustrates an interaction of a CAM row on the match line. Each cell is capable of providing some current onto the match line. In FIG. 1 the match line is labeled match to identify that a match indication in the described circuit results from a "low" voltage signal on the match line. Current or voltage sensing detects this current. Regardless of the sensing method, several features remain consistent: the match line has a high relative capacitance since it is shared by all cells in a row; the gain of the sense amplifier is large to achieve high speed; the amplifier is relatively small and consumes relatively low power since many will be active simultaneously.
Designers combine bipolar transistors to perform the sensing with CMOS devices for loads and biasing network to meet the above requirements. An exponential relationship between a base-emitter voltage and a collector current of a bipolar transistor allows for very fast sensing. Also, due to higher transconductance, bipolar transistors make superior amplifiers compared to MOS devices The higher transconductance makes the bipolar transistors more sensitive to small voltage variations at their input. At the same time, use of CMOS transistors for a remainder of the sense amplifier circuit reduces the noise, power dissipation, and size of the circuit.
Typical applications for CAMs are in devices such as catalog memories where a CAM interfaces with a RAM array. In order for the sense amplifier to interface to a row line driver of one of these RAM arrays, it must provide CMOS voltage levels. Designers achieve CMOS voltage levels by either designing an amplifier with a low output voltage swing followed by a voltage level translation to CMOS levels, or by performing level translation and sensing in one stage. The first method slows down the detection time since fast level translation circuits are difficult to design. The second method requires the collector voltage of the bipolar device (output of the amplifier) to swing full CMOS levels which produces saturation of the bipolar device. Since switching times of bipolar devices increase significantly in their saturation region, avoiding saturation requires additional complicated circuitry.
Furthermore, the base-emitter voltage of a bipolar transistor has a negative temperature coefficient while the threshold voltages of the MOS devices do not. Therefore temperature and process shifts would either make a CMOS biasing network for the bipolar amplifier nonfunctional, or require sufficient safety margins to limit its applications.